A pressure-sensitive recording sheet is generally composed of (1) a combination of an upper leaf (or upper paper) comprising a support having coated thereon a layer containing microcapsules prepared by dissolving substantially colorless electron donating dye(s) (hereinafter referred to as "color former(s)") in a suitable solvent and microcapsulating the oil droplets of the solution thus formed (hereinafter such microcapsules containing the oil droplets are referred to more simply as "microcapsules") and a lower leaf (or lower paper) comprising other support having coated thereon a layer of electron accepting compound (hereinafter referred to as developer); or (2) a combination of the above-described upper and lower leaves and an intermediate leaf (or intermediate paper) comprising a support having a layer of microcapsules on one surface thereof and a layer of a developer on the other surface thereof; or (3) a sheet comprising a support having microcapsules and a developer on one surface thereof in a state of being containing in one layer or in two disposed layers respectively; or (4) a sheet comprising a support containing one of microcapsules and a developer and having the other thereon as a layer coated thereon.
These pressure-sensitive recording papers are described, for example, in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,730,457, 3,418,250, etc.
However, conventional microcapsule sheets for pressure-sensitive copying have a serious disadvantage that they tend to form fog during printing.
A synthetic polymer latex is typically used as an adhesive and also is often used as a binder for developer layers in pressure-sensitive copying paper (pressure-sensitive recording sheet). On the other hand, a synthetic polymer latex is not generally used for a microcapsule sheet containing microcapsules prepared by a coacervation of gelatin for reasons such as that the microcapsule films become weak, etc., but a pressure-sensitive copying microcapsule sheet containing microcapsules composed of a melamine-formaldehyde resin film or a urea-formaldehyde resin film using a synthetic polymer latex as a binder is disclosed in Japanese Patent Application (OPI) No. 77589/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
However, the simple use of a synthetic polymer latex as a binder does not show the effect of preventing the occurrence of fog during copying. The inventors have found that it is necessary for preventing the occurrence of fog during copying of a pressure-sensitive copying paper to use a synthetic polymer latex having an oil swelling degree with respect to diisopropylnaphthalene of at least 50% as a binder for the microcapsule sheet in synthetic polymer latexes.